WILMINGTON, Del. -- Highway engineers say a crucial bridge on the Eastern Seaboard’s interstate highway system could imperil drivers if traffic is allowed back on it.

The bridge, near Wilmington, Delaware, was closed Monday when its support pillars where found to be tilting. The bridge won’t reopen anytime soon, highway officials said Tuesday, and the 90,000 vehicles that cross it every day are being diverted onto the main north highway, I-95, further overloading one of the most crowded arteries in America.

Engineers say ground under the pillars moved and caused the supports to tilt on the Interstate 495 bridge. Officials said they believe the bridge over the Christina River is stable enough to support itself, but that re-opening it to traffic could overload the structure.

Tripp Shenton, an expert on bridge superstructures and chairman of the Department of Civil and Environmental Engineering at the University of Delaware, said it would be tough to speculate on how close the bridge might have been to collapsing while carrying traffic.

"I don’t know that anybody at this point would be able to answer that, just because they don’t know how quickly this movement has occurred," he said.

Four pairs of 50-foot columns have been found to be leaning, with the top of one column roughly two feet out of line with the bottom. The columns are leaning by as much as 2.4 degrees, or 4 percent, from vertical. The whole bridge needs to be inspected, officials say.

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The closing forces more traffic onto I-95, which runs through downtown Wilmington, and already is heavily clogged during the morning and afternoon rush hours. Engineers are trying to find out the exact cause of the problem and there is no exact timetable for the bridge to reopen.

"It’s not going to be open anytime soon," said state Transportation Secretary Shailen Bhatt said in an interview Tuesday with The Associated Press.

An AP analysis of more than 600,000 bridges last year showed that more than 65,000 were classified as "structurally deficient" and more than 20,000 as "fracture critical." Of those, nearly 8,000 were both -- a combination of red flags that experts say indicate significant disrepair and similar risk of collapse.

The now-closed Delaware bridge was classified as "fracture critical." A bridge is deemed that when it doesn’t have redundant protections and is at risk of collapse if a single, vital component fails.

The problem with the bridge was discovered by crews working on an unrelated project.

Two employees of private geoscience consulting company Duffield Associates saw some cracking in the soils around a dirt pile, then looked at the columns and noticed that they appeared to be tilting, said the firm’s chairman, Jeff Bross.

"They were significant cracks," Bross said.

Bhatt could not say whether the dirt pile was there legally but noted that part of it appears to be in DelDOT’s right of way.

As for the detours and traffic, the transportation department has sent out notices from Maine to Florida, telling people the best routes to take to avoid the bridge.

Joe Erthal heard about the closure Tuesday morning while heading to Maryland from Philadelphia, where he lives. Driving home that afternoon, he got mixed up while trying to bypass the closure and mistakenly ended up on I-495.

"I guess we’re just going to have to find another way around ‘til everything’s fixed again," Erthal said while consulting a map app on his cellphone for directions.

Bridge collapses are not common, but some of the more prominent ones in recent history took place on fracture critical spans. That includes the Interstate 5 bridge that collapsed in Washington state last year, the 2007 failure of the I-35W bridge that killed 13 people in Minneapolis and the 1983 collapse of the I-95 bridge over the Mianus River in Connecticut. .

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